1. Field of Invention
The invention relates to a driving apparatus for a wind-driven generator. More particularly, the invention relates to a pitch driving apparatus used to adjust the pitch angle of the blade of a wind-driven generator.
2. Related Art
During the recent years, the output power of a wind driven generator has been increased to more than 5000 kW, and the length of the blade and tower height are extended over tens of meters. These factors contribute to an increased challenge in maintenance and a higher cost of the system. On the other hand, to reduce the cost of power generation and ensure the operational profit, the service life of a large wind driven generator is usually designed to be 20 years, requiring an extremely high reliability of parts or subsystems.
Currently, a large variable speed wind-driven generator usually adopts a variable speed generator with pitch adjustment mechanism to adjust the blade for limiting the wind energy obtained by the wind driven-generator, so as to control the output power of the wind-driven generator. The pitch adjustment mechanism may be a hydraulic system or an electric system, and hereinafter we refer only to the electric one and is termed as pitch drive. The pitch drive includes a pitch motor and a motor drive. The pitch drive is configured in a rotating hub and coupled to a corresponding blade through a reduction gear box. Thus, the pitch angle of the blade is adjusted by the pitch motor.
When the wind speed is between a rated wind speed and a cut-out speed, the pitch drive must adjust the pitch angle of the blade timely and accurately to limit the output power of the wind-driven generator to a rated value. When the wind speed exceeds the cut-out speed, the pitch drive adjusts the pitch angle of the blade to a position of 90° (also referred to as a feathered position) to prevent the wind-driven generator from being damaged by the strong wind. Therefore, the pitch drive is a critical subsystem in the variable speed wind-driven generator significantly. The output performance and the security of the wind-driven generator mostly rely on the configuration of the pitch drive. Because the blade of the large wind-driven generator can be hit by lightning, and the wind speed changes at any time, the pitch drive is usually in a load state with large variation. Thus, the failure of the pitch drive cannot be completely avoided and it is usually required that the pitch drive should have a redundancy design for consideration of the system reliability. The redundancy design enables the blade to rotate back to a feathered position (usually 90°) when the pitch drive fails, such that the wind-driven generator is in a minimum load state until being recovered.
The conventional pitch drive usually adopts a DC motor to serve as the pitch motor. This is because the DC motor has excellent servo control performance and in addition, once the pitch drive fails, the DC motor can impel the blade to rotate to the feathered position by using the power of a backup battery and hence meet the redundancy requirement easily.
European patent publication No. EP1664527 (also published as AU2003267035 and CN1860292) discloses the most typical redundancy design which has one more set of pitch drive installed in the hub as the redundant pitch drive. When the pitch drive fails, the redundant pitch drive replaces the failed one to maintain the continuous operation of the system.
Germany patent publication No. DE10116011 discloses a redundancy design having several pitch drives (for example three). Each pitch drive impels one motor to drive the same blade to rotate. As long as one of the drivers or motors fails, the remaining drivers drive the blade to a predetermined pitch angle (pitch position). Then the generator is shut down for reparation. The similar redundancy design of using more than two motors to drive the same blade to rotate has been published in Germany patent DE10140793 (patent family: EP1286048 and U.S. Pat. No. 6,783,326), and EP1647708 (patent family: US2006083615 and CN1782369). In EP1647708, in addition to a driving apparatus in which two motors (including drivers) are coupled with a reduction gear box, a spring transmission apparatus is added to provide assistant torque.
The most prior art adopts a DC motor as the one of the core elements of the blade pitch drive. Regular examination and reparation are required for maintaining the commutators and brushes of the DC motor. The heat dissipation is a concern in the DC motor because its armature winding is located on a rotation portion. Further, no ventilating or cooling system is disposed in the wind generator hub. Under heavy load conditions, accordingly the winding is easily overheated and the pitch motor might be therefore burnt out. In view of the above-mentioned disadvantages of the DC motor applied to a large scale wind-driven generator, the operation maintenance requirement and cost are relatively high. Although the above-mentioned published European patent EP1664527 adopts an AC motor, the similar redundancy design still has large complication or higher fabrication cost, and requires more backup batteries.